Method and device for subsea dredging

ABSTRACT

Method and device for moving subsea rocks and sediments, also at significant depths, e.g. in connection with removal of protective rocks around subsea installations, where maintenance is to be conducted. The device comprises a rigid or at least partly flexible tubing ( 5 ) thorough which the masses ( 14 ) may be transported with the aid of a pressure gradient produced by an ejector nozzle ( 11 ) arranged externally in relation to said tubing. The nozzle ( 11 ) is fed with water from a water pump ( 12 ). The device further comprises a chassis (F) adapted to be transported along the (sea) bottom. The required power is arranged to be supplied through a cable ( 3 ) from the surface, while the tubing ( 5 ) preferably is arranged to be remotely controlled by a manipulator ( 9, 9″ ).

[0001] The present invention relates to a method of the kind describedin the preamble of claim 1. The invention further relates to a deviceaccording to the preamble of claim 3 for conducting said method.

BACKGROUND

[0002] For work at subsea oil and gas installations or in connectionswith such installations, e.g. maintenance work, there is often a need tomove rocks and particulate material that partly covers the bodies thatare to be repaired. It can be pipelines, valve housings and the like.

[0003] In a similar way a need may occur to remove sediments inconnection with new installations on the sea bottom, or for removal ofcollected drill cuttings at platforms or the like.

[0004] Similar needs may also occur in connection with subsea work, likeharbour works or work at barrage or quay structures.

PREVIOUSLY KNOWN TECHNOLOGY

[0005] The most common way to remove sediments in connection with subseawork, is by utilizing large “fans”, large and heavy suction devices witha high power consumption and specially designed excavators.Disadvantages are that they require a lot of power and/or otherresources, they require large surface vessels, have a limitedversatility, are as good as stationary, or they are not at all suitedfor deep waters.

[0006] NO patent No. 302.043 describes a dredge designed for subseaoperations, especially to remove or move drill cuttings, comprising amotor, a pump device and an ejector, where the motor is designed to runthe pump which in its turn provides a stream of water to the ejector,which is positioned in a tubing through which the cuttings or the likeis supposed to be transported. The apparatus is designed to rest on thesea bottom and to receive energy from the surface, while the inlet endof the tubing is supposed to be moveable e.g. with the aid of a remotecontrolled mini submarine, a so called ROV.

[0007] This apparatus is not suited to move sediments with relativelylarge rocks, mainly because the pipeline has an effective loss ofdiameter due to the ejector's design and position. Further it has ageographically very limited work range as it is designed to rest at thesea bottom, even though the pipeline is designed to be somewhatmoveable.

[0008] Japanese patent applications Nos. 043 25 799 A and 043 25 800 Adescribes an ejector pump system where the ejector is positioned mainlyoutside the pipeline so that the ejector does not reduce the effectivediameter of the pipeline. From the abstract of these patent applicationsit is not possible to see what kind of utilizations these systems aremeant for. Neither are there any indications of dimensions or powerrequirements for these systems.

OBJECTS

[0009] It is an object with the present invention to provide a methodfor transportation of rocks and sediments under water, especially atdeep waters.

[0010] It is a particular objective to provide a method fortransportation of rocks with a typical maximum diameter of 250-500 mm.

[0011] It is a further object to provide an apparatus for performingsaid method, which apparatus should be versatile in its use, especiallyin the way that it should be easy to move around down at the sea bottom.

[0012] It is a still further object to provide such an apparatus that iseasy to control, and which does not require more energy than what may besupplied from the surface, e.g. through a conventional electric cable.

THE INVENTION

[0013] These and other objects are achieved by the method according tothe invention as defined by claim 1. Preferred embodiments of theinvention are disclosed by the dependent claims 2-5.

[0014] The mentioned objects are also achieved by a device as defined byclaim 6. Preferred embodiments of the device are disclosed by thedependent claims directed to the device.

[0015] Below a more detailed description of a device according to theinvention is given with reference to the accompanying drawings, where:

[0016]FIG. 1 is a schematic drawing of a first embodiment of theinvention,

[0017]FIG. 2 is a schematic drawing of a second embodiment of theinvention,

[0018]FIG. 3 is a simplified schematic drawing of a third embodiment ofthe invention,

[0019]FIGS. 4a-c shows details of a device according to the inventionaccording to any one of the embodiments shown in FIGS. 1-3.

[0020]FIG. 1 shows a device 1 designed to move on the sea bottom S withthe aid of belts 2 powered from the surface through a cable 3. Thedevice comprises a tubing 5, preferably with a flexible section 8, saidtubing having an inlet end 6 and an outlet end 7. To the tubing anejector nozzle 11 is attached, said nozzle being supplied with waterfrom a pump 12 powered by an hydraulic unit 13. All of said equipmentare supported by a chassis F which again is supported by the belts 2. Itis preferred that the tubing 5, when it includes a flexible section 8,further comprises a manipulator 9 which is able to move the tubingwithin certain degrees of freedom. In FIG. 1 the manipulator 9 consistsof a multi-link arm controlled by means of an hydraulic unit 16. Thedevice is adapted to transport sediments 14 including rocks of a size upto the diameter of the tubing 5 from one site to another, by thepressure gradient in the tubing set up by the ejector nozzle 11,providing a “vacuum from left to right in the drawing.

[0021]FIG. 2 shows an alternative embodiment of the invention. By thisembodiment there is no power to the wheels or belts, as the device issupported by freely moving wheels 2′ capable of being turned in severaldirections and preferably in any direction. The drawing shows 2 wheelswhile it is understood that at least two other wheels are hidden behindthese two. Most typically the device in this embodiment has 4 wheels,but it may also have e.g. 3 or 5 wheels. As an alternative to freelymoving wheels, freely moving belts may be utilized.

[0022] By the device according to FIG. 2, the manipulator 9′ consists ofa remotely operated vehicle (ROV) controlling the tubing 5 and, if thesea-bottom so allows, the ROV may pull the entire device 1 in a desireddirection. It is to be understood that the freely moving wheels 2′ neednot have the shown shape, they may have any form suited for subseatransportation.

[0023]FIG. 3 shows a further embodiment of the device according to theinvention, an embodiment that may be seen as a variation of theembodiment of FIG. 2. FIG. 3 is simplified and does not show all thefeatures of FIG. 2. The central aspect of this embodiment lies in thedetails indicated by the reference numeral 2″, which may be denoted“water cushions” (cf. air cushions of a hovercraft), which may cause thedevice to float just above the sea level. The so-called water cushionsare supplied with water from a powerful water pump, for instance thepump that feeds the ejector nozzle 11. In the drawing this is shownschematically in the form of a particular supply conduit 18 from thepump 12. Movement of tubing 5 and possibly of the entire device 1 may asshown in FIG. 2, be effectuated by means of a pulling force from an ROVthrough the tubing 5.

[0024]FIG. 4 shows details at the inlet end 6. The FIG. 4a shows thatthe outer part (mouth piece) of the inlet end 6 comprises telescopicunits 21 may be pulled or pushed out. FIG. 4b further shows that themouth piece may comprise an annulus “lance” 21 which is hollow and whichis able to flush water through a plurality of openings 22 inwards aswell as outwards relative to the mouth piece, so that the inlet end assuch becomes shielded and not so easily will become packed when themouth piece is pressed into the sediments. The water is fed to the lancethrough conduit 23 which may communicate with e.g. the water pump 12 oranother suitable water pump.

[0025] At the inlet mouth piece 10 of the tubing 5 there may also beprovided a nozzle (not shown) for backflushing of rocks etc. that mightget stuck in the mouth piece.

[0026] Further it is preferred that the inlet mouth piece 10 is roundedand that the cross-section of the tubing is constant, and that any bendon the tubing 5 has sufficiently large radius to ensure that rocks willnot get stuck. It is further preferred that the outlet end 7 of thetubing is shaped as a diffusor, as this reduces the frictional lossthrough the tubing. The device according to the invention may bemanufactured mainly in a plastic material with a density close to thatof water, so that it is easy to support.

CALCULATION EXAMPLE

[0027] In the following example of utilization there is an assumption ofone or two water pumps each powered by a motor of 75 kW. It is assumedthat the tubing has an internal diameter of 300 mm. In the case of twopumps there is also conducted calculations for a 500 mm tubing. Furtherdata are given in the table below. Motor power (axle-) kW 75 150 150Power efficiency % 80 80 80 Internal diameter mm 300 300 500 Length(inlet-outlet) m 15 15 12 Speed prior to mixing m/s 5.8 7.4 5.9 chamberRequired speed m/s 4.4 4.4 5.7 Motive power m 2.5 4.2 1.8 (liftingheight) of which inlet loss is m 0.3 0.6 0.4 frictional loss is m 1.42.3 0.7 outlet loss is m 0.7 1.3 0.7 Ca. capacity transport rockstons/hour 70 120 100

PRACTICAL EXAMPLE

[0028] A commission conducted shows that the invention works inpractice. During the summer of 1999, 1500 m³ (d_(max)=ca. 150 mm) ofrocks were moved with the aid of a corresponding ejector mechanism,carried by a remotely operated vehicle, ROV. The commission wasconducted in Tengsfjorden, by an oil pipe at a depth of 540 m below sealevel. For powering the water pumps, two hydraulic engines with a totaleffect of approx. 24 kW were used. The tubing was 10 m long and had aninternal diameter of 250 mm.

[0029] During 26 effective work hours 1500 m³ of rocks were moved, whichcorresponds to a capacity of 60 tons/hour. Only a minimal wearage wasobserved on the tubing in PE plastic. Later, several successful taskshave been performed with this technology.

[0030] In March 2000 the present invention was utilized at the Draugenfield, at a depth of 300 m. The commission was carried out from the boatSeaway Kingfisher. 5 m length of a pipeline was uncovered during 40minutes before the commission had to be interrupted. This corresponds to20 m³ mass or 45 tons/hour. Considering that the rocks were moved from aregion where frequent re-locations of the device was required, theresult was very satisfying. A 75 kW pump and a tubing of 15 m with aninternal diameter of 300 mm was utilized.

[0031] The drawings and the examples are merely illustrations of theinvention, which is only limited by the subsequent claims.

1. Method for moving subsea rocks and sediments, also at significantdepths, e.g. in connection with removal of protective rocks aroundsubsea installations, where maintenance is to be conducted, utilizing arigid or at least partly flexible tubing through which a pressuregradient is produced by arranging an ejector nozzle externally inrelation to said tubing and feeding water from a water pump to saidnozzle, characterized in placing the tubing, the ejector and the pump ona chassis designed to move on the sea bottom, supplying the requiredpower through one or more cables form the surface and preferablyremotely controlling the tubing with a manipulator adapted to thepurpose.
 2. Method as claimed in claim 1, characterized in providing thechassis with belts or wheels and move it along the sea bottom with powerto the belts and/or the wheels respectively.
 3. Method as claimed inclaim 1, characterized in that the manipulator is an hydraulicallycontrolled multi-link arm.
 4. Method ac claimed in claim 1,characterized in providing the chassis with belts or wheels that areable to turn freely in several directions and using an ROV as themanipulator and that the ROV is utilized for moving the chassis as wellas for controlling the tubing.
 5. Method ac claimed in claim 1,characterized in furnishing the chassis with “water cushions” that arefed with water from one or several water pumps enabling the chassis tofloat above the sea bottom using an ROV as the manipulator, and that theROV is utilized for moving the chassis as well as for controlling thetubing
 6. Device for moving subsea rocks and sediments, also atsignificant depths, e.g. in connection with removal of protective rocksaround subsea installations, where maintenance is to be conducted,characterized in that the device comprises a rigid or at least partlyflexible tubing (5) through which the masses (14) may be transportedwith the aid of a pressure gradient produced by an ejector nozzle (11)arranged externally in relation to said tubing (5) said nozzle being fedwith water from a water pump (12), a chassis (F) adapted to betransported along the (sea) bottom and that all required power isarranged to be supplied through cable (3) from the surface, and that thetubing (5) preferably is arranged to be remotely controlled by amanipulator (9, 9′).
 7. Device as claimed in claim 6, characterized inthat the manipulator (9, 9′) is an hydraulically controlled multi-linkarm (9).
 8. Device as claimed in claim 6 or 7, characterized in that thechassis (F) is supported by belts (2) or by wheels (2′) and is arrangedto be transported along the sea bottom by power to the belts (2) and/orthe wheels (2′) respectively.
 9. Device as claimed in claim 6,characterized in that the manipulator (9, 9′) is an ROV (9′) that iscontrolled principally independently of the device (1) as such. 10.Device as claimed in claim 9, characterized in that the chassis (F) issupported by freely turning belts (2) or wheels (2′) that possibly maybe turned in any direction, and is arranged to be transported along thebottom with the aid of a pull force exerted by the ROV (9′) through thetubing (5).
 11. Device as claimed in claim 9, characterized in that thechassis (F) is provided with “water cushions” (2″) facing the sea bottomfed with water from the pump (12) or by specially adapted pumps (notshown), so that the chassis (F) may partly float above the bottom (S)and is arranged to be transported along the bottom with the aid of apull force exerted by the ROV (9′) through the tubing (5).
 12. Device asclaimed in claim 6, characterized in that the tubing (5) at its inletend (6) is telescopically extendable.
 13. Device as claimed in claim 6,characterized in that the inlet end (6) of the tubing (5) comprisesmeans (21) for penetration and pressurized water flushing (22) designedto disintegrate compact or hardened sediments in order to improve thesuction capacity of the device.
 14. Device as claimed in claim 6,characterized in that close by the inlet mouth piece (10) there isarranged a nozzle (not shown) for backwards flushing of rocks etc. thatmight get stuck in the mouth piece.
 15. Device as claimed in claim 6,characterized in that the inlet mouth piece (10) is rounded and that thecross-section of the tubing (5) is constant, and that any bend of thetubing (5) has sufficiently large radius to ensure that rocks can notget stuck therein and that the outlet end (7) is shaped as a diffusor(to minimize the friction loss through the tubing).